1. Field of Invention
The present invention pertains generally to variable flow reciprocating piston respirators and more particularly to variable flow reciprocating piston respirators with oxygen enrichment.
2. Description of the Background
Many times, patients who use respirator devices require a respirator output flow which is oxygen enriched. For variable flow reciprocating piston respirator systems, this requires the addition of oxygen to the system after, or downstream from, a piston pump which produces a variable flow output. The necessity for downstream addition of oxygen to the system is a result of various health and safety hazards associated with insertion of oxygen into the piston pump. For example, contamination of the oxygen supply can occur by insertion of oxygen into the piston pump. Additionally, insertion of a pure source of oxygen into an electromechanical device, such as a piston pump is considered extremely dangerous because of the hazards associated with fire or explosion as a result of ignition of the oxygen in the electromechanical device. Hence, government regulations have prevented insertion of an oxygen source into the piston pump of reciprocating piston respirators where elaborate sealed chambers are not provided in the devices.
To overcome these problems, conventional oxygen enrichment respirator devices have added oxygen to the respirator system downstream from the piston pump. Oxygen can be supplied from a compressed oxygen tank or other source of oxygen. The problem with such systems, however, is that the oxygen source supplies a constant flow of oxygen to the system while the reciprocating piston respirator provides a variable flow. Consequently, the percentage of oxygen enrichment over a single inspiration stroke varies greatly because of the variable flow output of the piston pump. In other words, if a sinusoidally shaped variable output flow is produced by the piston pump during a single inspiration stroke, the percentage of oxygen is extremely high during the beginning and end of the inspiration stroke, and relatively low during the middle of the stroke.
The problem of mixing two gases to obtain a substantially constant proportion of the gases has been addressed in a number of different technical arts. Pre-examination patentability searches were performed which uncovered the following patents.
______________________________________ U.S. Pat. No. Inventor Issue Date ______________________________________ 2,403,508 Deming 7-9-46 3,308,817 Seeler 3-14-67 3,675,649 Basham et al. 7-11-72 3,896,837 Robling 7-29-75 4,204,536 Albarda 5-27-80 4,215,409 Strowe 7-29-80 4,215,681 Zalkin et al. 8-5-80 4,345,612 Koni et al. 8-24-82 4,380,233 Caillot 4-19-83 3,973,579 Ollivier 8-10-76 3,605,785 Kobritz 9-20-71 3,882,882 Preisig 5-13-75 4,085,766 Weigle et al. 4-25-78 4,064,891 Eberhardt 12-27-77 4,340,044 Levy et al. 7-20-82 4,336,590 Jacq et al. 6-22-82 3,385,295 Beasley 1-9-68 4,127,121 Westenskow et al. 4-28-78 3,362,404 Beasley 1-9-68 3,734,092 Kipling 4-22-73 3,834,383 Weigl et al. 9-10-74 4,044,763 Bird 8-30-77 4,425,805 Ogura et al. 1-17-84 ______________________________________
U.S. Pat. No. 3,973,579 issued to Ollivier discloses an apparatus for controlling the ratio of the flow of two gases. Unit 10 senses the flow rate of a gas in an initial portion 23 of conduit 20 by coupling the gas in the initial portion 23 to the chamber 30 via passage 28. Flexible diaphragm 33, sensing the pressure in chamber 30 and hence the flow rate of the gas in the initial portion 23, controls the amount of gas introduced into the downstream portion 27 of conduit 20 from source 44 via passage 29.
U.S. Pat. No. 4,340,044 issued to Levy et al. and U.S. Pat. No. 4,366,590 issued to Jacq et al. both disclose ventilators using microprocessor control. In particular, Levy et al. discloses a microprocessor for controlling the ratio of oxygen to air in a ventilator. Levy et al. does not add oxygen as a function of the flow rate of the gas, but rather, premixes the air and oxygen in a predetermined ratio.
U.S. Pat. No. 3,385,295 issued to Beasley discloses an apparatus for use in administering intermittent positive pressure breathing therapy which uses an exhalation valve 90.
U.S. Pat. No. 4,127,121 issued to Westenskow et al. discloses an oxygen and anesthesia delivery device having an oxygen sensor 3 in a feedback loop to control the amount of oxygen injected into the main breathing line for maintaining oxygen concentration constant in the main breathing line. Westenskow, et al. adds oxygen downstream from bellows 11. Oxygen sensor 3 produces a signal to control oxygen pump 9. The oxygen and anesthesia delivery system illustrated by Westenskow, et al. comprises a closed-loop system wherein the patient 1 depletes the oxygen level during breathing. Oxygen sensor 3 produces a signal to operate oxygen pump 9 to replace oxygen depleted by the patient. Hence the nitrogen oxide and oxygen are substantially premixed in the system, as illustrated by Westenskow.
U.S. Pat. No. 3,605,785 issued to Dobritz, U.S. Pat. No. 3,882,882 issued to Preisig, U.S. Pat. No. 4,085,766 issued to Weigl, et al., and U.S. Pat. No. 4,064,891 issued to Eberhardt all disclose an apparatus for maintaining the mixture of two gases at a substantially constant ratio. The remaining patents are not as pertinent as those described above.
The above cited patents primarily disclose mechanical devices for mixing two different types of gases to maintain a substantially constant ratio by sensing differential pressure between the gases and adjusting flow based on the differential pressure. Such devices have a slow response time, are inaccurate in operation and adjust the flow in a mechanical feedback system based upon previous information, rather than looking forward to the next segment to adjust the flow based upon a desired mixing ratio for a subsequent time interval. Although Levy et al. and Jacq et al. disclose microprocessor control of ventilators and devices for controlling gas flow, the Levy et al. device does not vary the oxygen supply to coincide with a variable flow rate and Jacq et al. merely incorporates a standard feedback control system for sensing differential pressures as do the mechanical control devices cited above. Jacq et al. does not disclose the maintenance of predetermined concentrations of either of the gases.
Consequently, the prior art fails to disclose a system for regulating the flow of gas from a pressurized source to maintain a predetermined concentration in a variable flow output using predictive servo control techniques. Although various feedback control techniques have been disclosed in the prior art, including both the mechanical and electronic control, such systems fail to provide the accuracy and response time necessary in applications as required in variable flow respirators.